Fix GPU-passthrough platform key for xenopsd

s/vgpu_pci_id/vgpu_pci in VM.platform when using full GPU-passthrough
(PCI-passthrough). The former is used for vGPU, the latter for full GPU.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #1429 from simonjbeaumont/fix-gpu-passthrough

Fix GPU-passthrough platform key for xenopsd